Currently there are two broad band carrier systems according to the 3rd generation time division duplex mobile communication system standard (refer to 3GPP related standard files). One is a Universal Telecommunication Radio Access Time Division Duplex (UTRA TDD) system of high chip rate (HCR) with a rate of 3.84 Mcps and the other is a TD-SCDMA system with a low chip rate (LCR) with a rate of 1.28 Mcps. Since the TD-SCDMA system uses an smart antenna technology and has an advantage of high efficient spectrum use, the TD-SCDMA system has obtained wide recognition and will be widely used in the coming one or two years. However, in the worldwide, people has stopped developing the UTRA TDD system.
In order to meet a requirement of a high density environment, a “multiple carrier TDD mobile communication system” is adopted. In this system, a base station is designed to work for multiple carriers and the multiple carriers serve one sector or one cell. Since the TDD mobile communication system uses multiple carrier frequencies in one cell or one sector, it has advantages of a flexible spectrum usage and the same properties of uplink and downlink transmission. Therefore, it is especially adaptable for asynchronous services and will be an optimum duplex mode for the beyond 3G mobile communication systems.
Applying the TDD technology in the public mobile communication system is an innovation of the 3G mobile communication international standard. Since it is an innovative standard, it has serious defects in the wireless transmission design for the HCR UTRA TDD system and it is also imperfect for the LCR TD-SCDMA system although it uses a lot of new technologies. The obvious defects are as follows.
The wireless frame structure of the HCR UTRA TDD system is to divide 10 ms into 15 slots with very small protecting interval between slots and Since the slots are averagely allocated, a cell with a big radius cannot be supported and it is difficult to realize the synchronous CDMA; since it uses the same cell searching technology with the CDMA FDD system and doesn't take into account that the system uses the same carrier frequencies in the downlink and uplink, i.e. this kind of cell searching technology can only be used in a single cell; the frame structure of the UTRA TDD system limits the usage of new technologies such as the smart antenna in the system, leading to the low usage ratio of the spectrum; the above problems also bring the defect of high cost of the system.
Currently, for the LCR TD-SCDMA system, China standard specifies that the CDMA TDD (LCR) mobile communication system works with multiple carriers such as carrier 1, carrier 2 and carrier 3 and the frame structure used by the base station in allocating channels is shown in FIG. 1, which is constituted by seven main slots from TS0 to TS6 and three special slots which are Downlink Pilot Time Slot (DwPTS), Uplink and Downlink Protecting Time Slot (G) and Uplink Pilot Time Slot (UpPTS) (in FIG. 1 only DwPTS is shown) and the base station is responsible for configuring the uplink and downlink slots for the main slots TS1 to TS6. Because of the LCR and limited radio resources on each carrier, when transmitting services with rates higher than 2 Mbps, the multiple carriers must be combined which leads to a complex terminal and high cost; in a mobile communication system with cellular architecture, a terminal may move to a position near 3-6 base stations. At this time, in each cell served by each carrier shown in FIG. 1, the terminal may receive different codes in the DwPTS of each carrier and the number of the codes may be the same with the number of the base station, such as 3-6 codes, which leads to the difficulty in cross cell switching for the terminal.
The common defects of the above LCR TD-SCDMA system and the HCR UTRA TDD system are that the length of the scrambling code is the same with that of the spreading code when the number of the spreading code is 16. Since if the length of the scrambling code is too short, there will be a very large peak signal power and a very bad in band amplitude and frequency characteristics, which will lead to an increase in the cost of RF components; problems related with the multiple carrier working mode in each cell and cellular networking are not fully taken into account; and transmission problems when the data transmission rate surpasses 2 Mbps are not fully taken into account.
In summary, related technologies of the TDD system in the 3G mobile communication standard need to be developed and improved so as to satisfy the requirements of the beyond 3G and further mobile communication services.